1. Field of Invention
This invention relates to methods and apparatus for making semiconductor devices. In particular, this invention relates to etching processes and etching apparatus.
2. Description of the Related Art
In etching processes for producing semiconductor devices, a film formed on a surface of a semiconductor substrate (wafer) is etched through a mask pattern formed on the film by a chemical reaction. Etching processes are classified into dry processes and wet processes. In dry processes, a process gas introduced into an etching chamber is excited by a plasma generated by an electric discharge, and a surface of a masked semiconductor substrate is exposed to the plasma to etch the film on the substrate. The process gas provided in the etching chamber is a gaseous mixture, which is formulated depending on the material of the film to be etched.
In a conventional process, for example, a process gas is introduced into an etching chamber by supplying a plurality of component gases into the chamber with controlled flow rates. A semiconductor wafer having a film to be etched and a mask pattern on the film is placed on a stage, to which a bias voltage is applied, and is irradiated with radicals and ions formed by plasma excitation of the process gas. The film is etched through the mask to form a desired pattern.
When the film to be etched has a multi-layered structure, appropriate etching parameters, such as pressure and composition of the process gas (i.e., kinds of component gases included in the process gas and/or ratios between the component gases), must be set for each of the layers. The plasma excitation of the process gas for the first layer is interrupted before changing the process gas in the chamber, and is resumed after a period necessary to stabilize the process gas for the second layer.
An example etching process is a process for etching a multi-layered structure of BARC (bottom antireflective coating)/WSi (tungsten silicide)/poly-Si layers formed on a SiO2/Si substrate. Such structure is typically etched to form a gate electrode of a MOS transistor. Typically, an O2-Cl2-Ar-based process gas is used for etching the organic BARC layer and an O2-Cl2-based process gas is used for etching the WSi layer. Further, a two-step etching process, including a main etch using an O2-HBr-Cl2-based process gas to etch a substantial thickness of the poly-Si layer, and an overetch using an O2-HBr-based process gas to clear residues remaining on the surface of the underlying SiO2 layer is employed.
When the step is changed from the BARC etching to the WSi etching, a total gas flow rate significantly changes due to termination of the Ar gas flow, and a total gas pressure in the chamber may become unstable. The plasma discharge is interrupted to prevent uncontrolled etching during this unstable period. When the step is changed from the WSi etching step to the poly-Si etching step, HBr gas is introduced into the process gas. The total pressure and the ratio between component gases in the process gas may become unstable due to an abrupt increase of HBr gas flow rate. The plasma discharge is interrupted until the process gas condition is stabilized. When the step is changed from the poly-Si etching to the overetching, the total gas pressure may become unstable because the total pressure of the overetching process gas is significantly higher than that of the main etching process gas. The plasma discharge is also interrupted to prevent uncontrolled etching of the underlying SiO2 layer.
The downtime for the temporary interruption of the plasma discharge significantly decreases the production efficiency. The downtime is approximately 15 seconds to 20 seconds for each change. For example, in a standard 0.35-μm process, the total downtime becomes approximately 45 seconds to 60 seconds due to these three interruption periods. This downtime occupies approximately one-third of the total etching time (approximately 180 seconds) for etching, for example, BARC/WSi/poly-Si=110/100/150 nm structure.
A hard mask of a SiO2 or SiN layer may also be used to form a gate electrode. In this case, a composite structure of BARC/SiO2 or SiN layer/WSi layer/poly-Si layer should be etched. The etching reaction of the SiO2 or SiN layer is significantly different from the etching reaction of the overlying BARC layer and that of the underlying WSi layer, and requires different etching species. Therefore, a process gas with a different chemistry, i.e., a process gas including a different main etchant gas, is need. More specifically, different from above described process gases for etching BARC or WSi layers including a chlorine-containing gas and an oxygen-containing gas as main etchant gases, a process gas including a fluorine-containing gas as a main etchant gas, for example, a CF4-Ar-based gas composition, a SF6-CF4-Ar-based gas composition, or a mixture of CHF3 gas with one of the CF4-Ar-based gas compositions is used.
To switch between such different process gases with different gas chemistries, a fairly long time is needed, and the plasma discharge must be interrupted for every change from one layer to the subsequent layer.
The interruption and subsequent resumption of the plasma discharge may also degrade the production yield. Particles generated during repeated processing in the etching chamber accumulate on the inner wall of the chamber. These particles are released from the chamber well and re-enter into the processing space when the process gas is introduced or when the plasma discharge is interrupted or resumed. The re-entered particles are electrically charged and float in the plasma, and may adhere to the surface of the semiconductor substrate when the discharge is interrupted. The particles adhered to the surface of the semiconductor substrate function as a mask for the subsequent etching step, resulting in patterning defects and decreased yield. Such defects will become a significant problem in the production of advanced semiconductor devices with reduced critical dimensions.